Field of the invention
The present invention relates generally to an ultrasonic system for detecting the presence of air in a fluid line, and more particularly to an apparatus which automatically loads the outlet tubing at the bottom of a disposable cassette into a notch in an ultrasonic sensor housing located on a main pump unit when the cassette is installed on the main pump unit.
In the past there have been two primary techniques which have been used to deliver drugs which may not be orally ingested to a patient. The first such technique is through an injection, or shot, using a syringe and needle which delivers a large dosage at relatively infrequent intervals to the patient. This technique is not always satisfactory, particularly when the drug being administered is potentially lethal, has negative side effects when delivered in a large dosage, or must be delivered more or less continuously to achieve the desired therapeutic effect. This problem results in smaller injections being given at more frequent intervals, a compromise approach not yielding satisfactory results.
Alternatively, the second technique involves administering a continuous flow of medication to the patient, typically through an IV bottle. Medication may also be delivered through an IV system with an injection being made into a complex maze of IV tubes, hoses, and other paraphernalia. With drop counters being used to meter the amount of bulk fluid delivered, many medications still end up being administered in a large dosage through an injection into the IV lines, although the medications may be diluted somewhat by the bulk fluid.
As an alternative to these two techniques of administering medication to a patient, the relatively recent addition of medication infusion pumps has come as a welcome improvement. Medication infusion pumps are utilized to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic the natural process whereby chemical balances are maintained more precisely by operating on a continuous time basis.
One of the requirements of a medication infusion system is dictated by the important design consideration of disposability. Since the portion of the device through which medication is pumped must be sterile, in most applications of modern medication infusion equipment some portions of the equipment are used only once and then disposed of, typically at regular intervals such as once daily. It is therefore desirable that the fluid pump portion of the infusion pump device be disposable, with the fluid pump being designed as an attachable cassette which is of inexpensive design, and which is easily installable onto the main pump unit.
It will be perceived that it is desirable to have a simple disposable cassette design to minimize the cost of construction of the cassette, using the minimum number of parts necessary in the design of the cassette. The design of the cassette must be mass producible, and yet result in a uniform cassette which is capable of delivering liquid medication or other therapeutic fluids with a high degree of accuracy. The cassette should include therein more than just a fluid pump; other features which have formerly been included in peripheral devices may be included in the cassette.
Such a system has been disclosed in all of the above-identified previously filed related applications. Of these applications, U.S. Ser. No. 128,121, entitled "Air-In-Line Detector for a Medication Infusion System," is hereby incorporated herein by reference.
An essential function of a medication infusion system is to avoid the infusion of fluid containing more than a minimal amount of air bubbles therein. Although steps may be taken to minimize the possibility of air bubbles being contained in a fluid which is to be infused to a patient, it is essential to monitor the fluid line before it reaches the patient to ensure that air bubbles remain in the fluid which is to be infused are detected. The detection of air bubbles in all fluids which are to be infused is therefore a critical design requirement.
One type of air-in-line detector which has been used in the past is an ultrasonic detector, which places an ultrasonic transmitter on one side of a fluid line and an ultrasonic receiver on the other side of the fluid line. Fluid is a good conductor of ultrasonic energy while air or foam is not. Accordingly, if there is an air bubble in the fluid line between the transmitter and the receiver, the signal strength will be greatly attenuated, and the presence of the bubble will be indicated. Examples of ultrasonic air-in-line detectors include U.S. Pat. No. 4,764,166, to Spani, and U.S. Pat. No. 4,821,558, to Pastrone et al.
The Pastrone et al. device has a projection from a disposable cassette which fits into an ultrasonic detector. Thus, the fluid path monitored in the Pastrone et al. device is within the fluid cassette. It is particularly desirable to monitor the fluid path as it leaves the cassette to determine whether or not there are air bubbles contained therein. The Spani device is designed to monitor fluid flow through a segment of flexible tubing, which tubing may be the fluid path as it leaves the cassette.
The Spani device will work quite well to detect the presence of air bubbles in the tubing, but is had at least two disadvantages in its operation. First, the tubing must be installed in the Spani device in a step additional to the installation of a cassette onto a main pump unit. In other words, there is no provision in Spani for the tubing to automatically be installed in the Spani device. This represents a disadvantage since an extra step is required by the operator of the system.
Secondly, the Spani device is extremely sensitive to placement of the tubing in the sensor. If the tubing is moved slightly, the path of ultrasonic energy could be disrupted, resulting in a false indication that there is air in the fluid line. Other than friction, there is no provision in the Spani device to retain the tubing in place in the sensor. Thus, while the Spani sensor operates well, it has certain disadvantages with regard to the installation and retention of the tubing in the sensor.
It is therefore the primary objective of the present invention to provide an ultrasonic air-in-line detection sensor for use with the outlet tubing from a cassette, with the tubing being automatically loaded into the proper position in the sensor when the cassette is installed on a main pump unit. The system of the present invention should thus load the tubing into the sensor, in the proper position, with no more effort required than the effort to load the cassette itself onto the main pump unit. It must be emphasized that the tubing has to be loaded correctly into the sensor in a highly repeatable manner.
It is also an objective that once the tubing has been properly loaded into the sensor that it be retained in the sensor in a manner making it highly unlikely that the tubing will be able to move with respect to the sensor. The tubing must thus be both properly loaded initially into the sensor, and retained in its proper position in the sensor to prevent false signals due to movement of the tubing in the sensor. In addition, it is an objective that the tubing be automatically removed from the sensor when the cassette is removed from the main pump unit without requiring any additional steps to be performed.
Despite the inclusion of all of the aforesaid features, the system of the present invention shall utilize a minimum number of parts, all of which parts are of inexpensive construction, yet which afford the assembled sensor and its mounting hardware the high degree of accuracy which must be retained. The system of the present invention must also be of a design which enables it to compete economically with known competing systems, and it must provide an ease of use rivaling the best of competing systems. The system must accomplish all these objects in a manner which will retain and enhance all of the advantages of reliability, durability, and safety of operation. The system of the present invention must provide all of these advantages and overcome the limitations of the background art without incurring any relative disadvantage.